US district judge James Cohn called Lucy Richards’ actions towards Leonard Pozner “disturbing” and condemned those who spread false claims about the deaths of 20 children and six adults in the attack in Newtown, Connecticut, five years ago.

“This is reality and there is no fiction. There are no alternative facts,” Cohn told Richards, 57, at her sentencing.

“You have the absolute right to think and believe as you so desire,” the judge said. “You do not have the right to transmit threats to another.”

For Pozner, who has become an activist devoted to combatting the conspiracy theorists that target family members of victims, the sentencing is a “powerful outcome” that he hopes will raise awareness of the serious harm that hoaxers cause.

“It draws a line,” he said, “and it shows people that this is not a game that’s online, and there are actual consequences for someone who steps over the line.”

Pozner has spent years using every tool at his disposal to scrub the online record of his son’s memory clean of videos suggesting that Noah and his surviving siblings and his parents are actors perpetrating a massive conspiracy against the American public. Pozner has made some progress in defending his family against these lies, but he said that countering hoaxers is still an uphill battle. Big tech companies such as Google, Twitter and Facebook are doing far too little to deter the conspiracies, harassment and hatred that flourish on their sites, Pozner said.

While these companies have been talking publicly about confronting fake news, “They really don’t do that much. They can be doing a lot more – especially Facebook. It really turns the other way.”

He said tech companies that allow the spread of conspiracies and hatred, including Islamic State propaganda, “have blood on their hands”.

Individuals are also failing in their responsibility to confront the conspiracy theorists in their own families, Pozner said.

“Everyone knows someone who follows these beliefs. Everyone has a cousin or an uncle or a relative when they have family gatherings will spend time talking about this.”

Most people “would just laugh it off: ‘He’s kind of a kook, he has weird beliefs, so glad we don’t have to see him till Thanksgiving or Easter’.” But these family kooks “may be hurting people, and harassing people, and spreading damaging messages. They have a responsibility to challenge them, not just to ignore it.”

“The internet is not a massive multiplayer online game that is just a game,” Pozner said. “People who are on the internet then go do things in the real world.”

The internet is not a massive multiplayer online game. People on the internet then go do things in the real world

Leonard Pozner

Pozner’s six-year-old son, Noah, was murdered alongside his first-grade classmates in Newtown on 14 December 2012. Pozner’s young son was one of 20 first-graders and six educators who were shot to death in a small suburban elementary school by a troubled local man with a military-style rifle in one of the most shocking mass shootings in US history.

But in the early stages of mourning their murdered children, the Pozners and other families of victims had to confront an extra burden: accusations that their children were not dead, and that the tragedy that upended their lives was all a fraud designed to undermine Americans’ gun rights.

Other family members of public shooting victims have described similar harassment. Chris Hurst, whose girlfriend, 24-year-old journalist Alison Parker, was shot to death on live television, said he has received messages accusing him of being a “crisis actor” paid by the government, and claims that Parker is still alive, got plastic surgery and now lives in Israel.

Richards, a 57-year-old from Florida, admitted in her guilty plea this week to being part of the active online community of Americans who believe that the Newtown shooting was staged and that the victims and their mourning family members are only actors.

She admitted to sending threatening messages last year to Pozner, telling him multiple times that death was coming to him “real soon”. According to court filings, Richards was angry at a news story she had read about Pozner’s interactions with Florida university professor James Tracy, another Sandy Hook denier, who was fired after the Pozners wrote an op-ed describing him as “chief among the conspiracy theorists”, a man who had “even sent us a certified letter demanding proof that Noah once lived”. (Tracy is suing the university for wrongful termination.)

Noah Pozner. Photograph: AP

In a statement that Pozner and his ex-wife, Veronique, submitted to the court before Richards’ sentencing, they wrote: “Losing a child, particularly in the manner we lost Noah, is a horrifying, life-changing event. Day after day, we wake up with the knowledge that we have to face the rest of our life without our boy. People like Lucy Richards, who torment victims of violent crime by denying and mocking their pain, are a sad reality.”

“Whatever demons drove her to threaten me, she needs to be held accountable for her crime,” the Pozners wrote. “Using the internet/social media to harass/persecute others is the dark underbelly of electronic communication, and we are glad that our justice system is holding those who subscribe to this form of harassment accountable.”

Court documents showed that Richards had significant mental health problems including obsessive compulsive disorder, anxiety disorder and agoraphobia, or fear of leaving one’s house, and she appeared in court in a wheelchair. An assistant to public defender Robert Berube, Richards’ attorney, said their office could not comment.

Richards apologized in a statement before she was sentenced to five months in prison followed by five months of home detention.

“I don’t know where my heart and head were that day, but they were not in the right place,” she said. “It was the worst mistake of my life and I am truly sorry.”

Pozner said he hoped Richards’ sentence might deter some of the people dabbling in conspiracy theories online and convince them to “refocus” before they were drawn in too deeply.

While he had no exact data, Pozner said he believes there are “tens of thousands” of conspiracy theorists globally who believe that mass casualty attacks such as Sandy Hook, the Boston Marathon bombing and the 2012 mass shooting at a movie theater in Aurora, Colorado, are hoaxes.

As more and more people join social media sites, Pozner believes the problem is growing more acute – and more people vulnerable to conspiracy theories are exposed to the “contagious addictive content” that spread hoaxes like viruses across the internet.

Pozner, who once listened to InfoWars’ Alex Jones before Jones became a Sandy Hook conspiracy theorist, said that people were drawn to hoaxes for different reasons.

Some “have nothing else going on in their lives and this gives them a sense of community, a sense of identity. It’s like a cult,” he said. Others are drawn to feeling “smarter” than the people around them who do not believe the conspiracy, thinking, “Oh, I’m awake – you guys are not awake.”

A woman visits a memorial to the victims of the Sandy Hook shooting. Photograph: Lucas Jackson/Reuters

Other people, such as the mothers of young children who spread Sandy Hook hoax theories, simply need to believe “that the world is not a fragile, unpredictable, ugly place where children can get blown up at a music concert”.

The HONR Network, the organization Pozner founded to hold hoaxers accountable for their behavior, now has about 300 volunteers who help monitor hoaxer content online.

It is a gruesome world to confront briefly, much less over and over again. Hoaxers, for example, will share videos of Boston Marathon bombing victims having their limbs blown off, and then debate whether the color of the blood in the footage is realistic, or look for evidence that the person in the footage whose limb was just blown off was already an amputee who is now faking a new injury.

The long fight to use copyright protections to remove family photographs of Noah and his surviving children from posts full of wild conspiracies has left him frustrated at tech companies, who he said often push back or simply refuse to take down content related to his young children.

“If you become the victim of a crime, shouldn’t you be protected by society somehow? Shouldn’t society hold you up when you’re kind of weakened? That’s not the case right now. If life has thrown you a curveball and you’ve been knocked to the ground, YouTube and Google and Facebook think that others should be allowed to kick you while you’re down as much as they like, and they’ll protect those people’s right to kick you.”

Tech companies have little incentive to take down content that they can advertise against – even when the content is false and harmful, he said. “It’s business, and these videos and posts are content.”

“Young people – they see this and they read it and because it’s allowed to remain online they have this idea, maybe there’s truth to it.”

While he sees it as positive that tech companies are beginning to acknowledge concerns about the use of their platforms to spread lies and hate, their efforts so far are “just placating everybody. They’re not really doing anything,” he said.

Facebook’s attempt to give users a way to flag fake news stories “feels like one of these walk buttons on a busy street that doesn’t do anything when you push it”.